Leak detection is an essential aspect of the quality control in the construction of aircraft structures such as pressurized crew areas and fuel tanks. To check the integrity of such structures during and after construction a variety of methods are used for example, the structure is pressurized with air and a soap film is applied to the area being inspected and the formation of bubbles is visually noted. Another method involves pressurizing the structure being tested with ammonia gas and spraying the surface being tested with a viscous liquid which contains an acid-based indicator which will change color if a leak allows ammonia gas to escape. Still another method of testing for leaks involves pressurizing with a mass spectrometer. Leaks in fuel tanks may also be detected using thermographic techniques. Another method of detecting fuel leaks in both partially assembled or assembled aircraft involves applying an oil red 0 dyed talc to the surface being tested. The talc is applied by mixing it with 1,1,1 trichloroethane and spraying the mixture onto the surface being tested. The trichloroethane quickly evaporates leaving a pink powder on the test area. This method has been found to be an effective indicator of fuel tank leaks, however, the material used in the test procedure has several undesirable characteristics. The dye oil red O is a petroleum dye that is soluble in hydrocarbon solvents such as 1,1,1 trichloroethane and JP-5 jet fuel and this solubility causes some of the dye to migrate from the talc carrier into the surface of the paint used on the test area thereby staining it. Another undesirable characteristic is the similarity of the relation of the oil red O dye with water and its reaction to jet fuel. If either fuel or water contacts the oil red O impregnated talc the color of the talc changes from pink to red thus making the test meaningless if water is present as a mist, spray, or as falling rain. This characteristic of the dye decreases the usefulness of the test in the field.
U.S. Pat. No. 3,368,994 discloses a test indicating permanent finish or paint for use with missiles which detects leaks of mixed rocket fuel. The finish is specifically directed to detecting leaks which are mixtures of the amine fuel component and the red fuming nitric acid oxidizing component of rocket fuel. A fuel leak is indicated when the finish color changes color.
U.S. Pat. No. 3,287,156 discloses a leak indicating system which utilizes an undercoating and an overcoating both of which change color when exposed to leaking fuels and oxidizers. The undercoating generally comprises cellulose base laquers and solvent type resins with phenosofranine dispersed therein for the undercoating and phenolphthalein dispersed therein for the overcoating. Again, a coating color change indicates a leak.
U.S. Pat. No. 3,266,920 also discloses a leak indicating paint for liquid propellants of a base type and an oxidizer. More specifically, the indicator paint comprises a paint matrix and dispersed solids which include phenolphthalien and a coloring agent which is readily bleachable in the presence of the oxidizer component of rocket fuel.
U.S. Pat. No. 4,326,981 discloses a water base leak detecting solution consisting of a nonionic oxyalkylated aliphatic alcohol surfactant, a small amount of a dual visibility dye which will not migrate or penetrate into a painted aircraft surface and a non-parasitic agent. This solution is introduced into the fuel tanks and external surfaces are then viewed under suitable lighting conditions to locate leaks.
U.S. Pat. No. 4,361,035 discloses a "depuddling agent" which is introduced into a tank being tested with a U.S. Pat. No. 4,326,981 type tracer solution, to make residual tracer solution miscible with subsequently introduced oil or petroleum based fuel.